Relationship between body collagen and urinary hydroxyproline excretion in young rats fed on a low-protein or low-calorie diet.

I . Hydroxyproline was determined in urine, carcass, and skin of young rats fed on lowprotein and low-calorie diets. 2. Protein and calorie deprivation in young rats resulted in a marked fall in the soluble as well as insoluble collagen content of the skin. 3. The collagen content of the carcass (excluding skin) was slightly greater in the deficient rats than in the weanling rats, but very much lower than in the well-fed rats. 4. In rats subjected to protein and calorie deficiency, urinary hydroxyproline was markedly reduced. In these rats, most of the reduction in urinary hydroxyproline was observed within I week of feeding the deficient diet. Protein deficiency had affected urinary excretion of hydroxyproline more severely than calorie deficiency. 5. The reduction of urinary hydroxyproline in the deficient animals can be attributed to decreased turnover rate of their body collagen. This apparent decrease in turnover rate is explained as being due to an increase in the proportion of collagen of slow turnover in these animals. 6. Protein and calorie deficiency had also resulted in the depletion of non-collagen nitrogen of both carcass and skin.